The FiveseveN exists because the P90 exists. The PS90 exists because the P90 exists and needed to be "Americanized". The P90 exists because FN Herstal wanted a bite of what it perceived as a burgeoning PDW market, believing that being among the first to make a serious offer in such a market would both create greater interest in the market and forgive using a proprietary caliber. The folks at FN Herstal are business-people, and have been most successful as a manufacturer first, innovator second. Why not add a profile to their name?
When you're discussing the merits (or lack thereof) of a caliber designed to compete with the MP-5 (which the P90's caliber was/is), you're discussing calibers that are competing with handgun ammunition. That's not a high hurdle to beat. Is anyone here confident enough in their weapon, of any caliber, to shoot just once?